Direct injection gasoline engine with stratified charge combustion and homogeneous charge combustion

ABSTRACT

An internal combustion gasoline engine has an air intake assembly which injects air through the cylinder head into the cylinder to generate swirl (horizontal vortex) flow (as opposed to tumble (vertical vortex) flow) for stratified charge combustion, and to generate tumble flow for homogeneous charge combustion. The piston includes a cavity combustion chamber at the top surface of the piston, the cavity combustion chamber having an increasing cross sectional area as the top of the piston is approached.

This application is a continuation of application Ser. No. 09/081,071,filed May 19, 1998 now U.S. Pat. No. 6,092,501. The entire contents ofthis Ser. No. 09/081,071 application are incorporated herein byreference.

BACKGROUND

The invention relates to a direct injection type internal combustiongasoline engine and particularly to improvements in direct injectiontype internal combustion engines which employ both homogeneous chargecombustion and stratified charge combustion.

A conventional engine injects gasoline into the air intake port upstreamof the combustion chamber. The air-fuel mixture is then transported tothe combustion chamber where it is burned. In contrast to thisarrangement, a direct injection gasoline engine has the fuel injectorlocated inside the combustion chamber so that the fuel is injecteddirectly into the cylinder.

The combustion process of a conventional gasoline engine is limited tohomogeneous charge combustion. A direct injection gasoline engine, onthe other hand, selectively uses both stratified charge combustion andhomogeneous charge combustion.

With stratified charge combustion, the fuel is injected into thecombustion chamber during the compression stroke. The objective is toposition a readily ignitable mixture in the vicinity of the spark plugwhile forming a surrounding air layer that contains little fuel. Thisprocess thus accomplishes stable combustion of an overall lean mixture.Lean combustion is accomplished in the stratified charge combustionprocess to improve fuel economy.

With homogeneous charge combustion, on the other hand, the fuel isinjected into the combustion chamber in the induction stroke. Similar toa conventional gasoline engine, the mixture is then uniformly mixed to astoichiometric ratio. This results in the generation of greater powerunder high-load operation.

In short, a direct injection gasoline engine employs stratified chargecombustion at low load for improved fuel economy and homogeneous chargecombustion at high load for greater power. This selective use of the twocombustion processes achieves both low fuel consumption and high poweroutput.

A variety of direct injection type internal combustion engines have beenproposed.

One of two different physical phenomenons may be employed duringstratified charge combustion. These phenomenons differ with respect tohow air flows in the cylinder during stratified charge combustion. Onetype is “tumble” flow wherein a vortex is created in a vertical plane ofthe cylinder. Such flow is shown, for example, in U.S. Pat. No.5,711,269 issued to Hideyuki Oda and others. The other type is “swirl”flow wherein a vortex is created in a horizontal plane of the cylinderduring stratified charge combustion.

This invention employs swirl (that is, horizontal vortex) flow duringstratified charge combustion.

A direct injection type internal combustion engine employing swirl flowduring stratified charge combustion is described, for example, in U.S.Pat. No. 5,553,588 issued to Takeshi Gono and others. The internalcombustion engine described in this patent is arranged as follows. Anon-circular cavity combustion chamber which is eccentric relative to apiston outer peripheral circle is formed at the top section of thepiston. A fuel injector valve is disposed to inject fuel toward thecavity combustion chamber near the upper dead center of the piston. Thecavity combustion chamber is a reentrant type so as to confine fuel andswirl therein. In other words, the cross-sectional area of thecombustion chamber is reduced toward the top of the piston. In order toproduce strong swirl in this cavity combustion chamber, one of a pair ofintake ports is arranged as a helical port, and an air control valve isprovided to open or close the other intake port.

The internal combustion engine in this patent is arranged such that,during lean combustion, the above-mentioned air control valve is closedso that fresh air is introduced only through the one helical port toproduce strong swirl inside the cylinder. Since this swirl is introducedinto the cavity combustion chamber with ascent of the piston, acombustible air-fuel mixture is formed and carried to near the sparkplug by injecting fuel into the cavity combustion chamber near the topdead center of compression. Accordingly, ignited combustion can beaccomplished by making ignition at a suitable timing.

However, the inventors of this invention have recognized severalshortcomings of this arrangement. In the above-mentioned arrangement ofthe piston, the cavity combustion chamber at the piston top section isnon-circular, such as generally triangle-shaped or cocoon-shaped, andtherefore a very strong swirl must be produced in the cylinder by usinga helical port in order to create swirl having a sufficient intensity.

However, using such a helical port increases intake air resistance athigh power output under the high load operation. To compensate for thisreduced output, a variable valve timing system is provided, whichcomplicates construction and increases cost.

Additionally, by forming the cavity combustion chamber in a reentrantshape, swirl and the air-fuel mixture is maintained inside the cavitycombustion chamber during stratified charge combustion. However, fueltends to stagnate in such a cavity combustion chamber during homogeneouscharge combustion, thereby degrading performance. Also, the requireddepth of the cavity combustion chamber increases the weight of thepiston, which in turn increases noise and vibration.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved direct injectionengine that overcomes the deficiencies in the prior art designs.

Another object of the invention is to provide a direct injection engine,which minimizes noise and vibration, is powerful, has improvedcombustion efficiency at all loads, has less pumping loss, and iseconomical to manufacture.

An engine constructed in accordance with the present invention employsstratified charge combustion (during low loads) and homogeneous chargecombustion (during high loads). The invention creates swirl flow (thatis, a horizontal vortex) during stratified charge combustion. The pistonbowl of the invention is shallow and has an increasing cross-sectionalarea as the top of the piston is approached. This design minimizes theweight of the piston (which minimizes noise and vibration) and improvescombustion efficiency by minimizing air-fuel stagnation duringhomogeneous charge combustion. The engine also employs an aerodynamicstraight port through the side of the cylinder head. This designimproves air induction efficiency and power and improves ease ofmanufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in further detail below with referenceto the accompanying drawings, wherein:

FIG. 1 is a vertical cross-sectional view showing an arrangement of adirect injection type internal combustion gasoline engine according to afirst embodiment of the present invention;

FIG. 2 is a bottom view showing a cylinder head in a state as viewedfrom a lower surface side;

FIG. 3 is a plan view showing a piston according to a first embodimentof this invention;

FIG. 4 is a cross-sectional view taken along the line IV-IV in FIG. 3;

FIG. 5 is a cross-sectional view taken along the line V-V in FIG. 3;

FIG. 6 is a plan view of a piston according to a second embodiment ofthis invention;

FIG. 7 is a cross-sectional view taken along the line VII-VII in FIG. 6;

FIG. 8 is an explanatory view showing flow within a cylinder under theaction of a piston during homogeneous charge combustion;

FIG. 9 is a characteristic graph showing an example of valve liftcharacteristics of an intake valve;

FIG. 10 is a characteristic graph showing another example of valve liftcharacteristics of an intake valve;

FIG. 11 is a vertical cross-sectional view showing an arrangement of adirect injection type internal combustion engine according to a thirdembodiment of the present invention;

FIG. 12 is a bottom view showing a cylinder head in a state as viewedfrom a lower surface side;

FIG. 13 is a plan view showing a piston according to the thirdembodiment of this invention;

FIG. 14 is a cross-sectional view taken along the line XIV-XIV in FIG.13;

FIG. 15 is a cross-sectional view taken along the line XV-XV in FIG. 13;

FIG. 16 is a plan view of a piston according to a fourth embodiment ofthis invention;

FIG. 17 is a cross-sectional view taken along the line XVII-XVII in FIG.16;

FIG. 18 is an explanatory view showing flow within a cylinder under theaction of a piston during homogeneous charge combustion;

FIG. 19 is a vertical cross-sectional view showing an arrangement of adirect injection type internal combustion engine according to a fifthembodiment of the present invention;

FIG. 20 is a bottom view showing a cylinder head in a state as viewedfrom a lower surface side;

FIG. 21 is a plan view showing a piston according to the fifthembodiment of this invention;

FIG. 22 is a cross-sectional view taken along the line XXII-XXII in FIG.21;

FIG. 23 is a characteristic graph showing the relationship between thedepth of a valve recess and the stability of stratified chargecombustion;

FIG. 24 is a characteristic graph showing the relationship between thepresence and absence of a projection section outer peripheral sectionand the HC emission amount;

FIG. 25 is a vertical sectional view of a direct injection type internalcombustion engine according to a sixth embodiment of the invention;

FIG. 26 is a view taken from along the line XXVI-XXVI of FIG. 25;

FIG. 27 is a front view of a spray recess;

FIG. 28 is a graph showing the relationship between the burningcondition and swirl control valve opening (depending on engine torqueand engine rotational speed);

FIG. 29 is a graph showing how swirl and tumble are generated dependingon the burning condition;

FIG. 30 is a graph showing the relationship between the fuel injectionangle, burning stability, and smoke generating condition, in the case ofatmospheric pressure;

FIG. 31 is a front view of an opening portion, in which the fuelinjection valve is contained, which is not provided with a spray recess;

FIG. 32 is a vertical sectional view of a direct injection type internalcombustion engine according to a seventh embodiment;

FIG. 33 is a view taken along the line XXXIII-XXXIII of FIG. 32;

FIG. 34 shows a modification, viewing a cylinder head from the cylinderside;

FIG. 35 shows a modification, viewing a cylinder head from the cylinderside; and

FIG. 36 is a plan view showing a piston according to the eighthembodiment of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Embodiment

The invention provides a direct injection type internal combustionengine which can realize stratified charge lean combustion without usinga helical port, and make stratified charge lean combustion andhomogeneous charge combustion compatible with each other.

FIGS. 1 to 5 illustrate a first embodiment of the invention.

First, the overall arrangement of a direct injection type internalcombustion gasoline engine employing a piston 4 of this invention willbe discussed with reference to FIGS. 1 and 2. Then, the design of thepiston 4 will be described in detail.

A plurality of cylinders 3 are disposed in line in a cylinder block 1. Acylinder head 2 is fixed to cover the upper surface of the cylinderblock 1. A piston 4 is slidably fitted in the cylinder 3. Additionally,a combustion chamber 11 is formed recessed in the cylinder head 2 and isarranged in a so-called pent roof type configuration, in which a pair ofintake valves 5 are disposed at one inclined surface 11 a of thecombustion chamber, while a pair of exhaust valves 6 are disposed atanother inclined surface 11 b. A spark plug 7 is disposed at thegenerally central position surrounded by the pair of intake valves 5 andthe pair of exhaust valves 6.

The cylinder head 2 is formed with a pair of intake ports 8 whichcorrespond respectively to the pair of intake valves 5 and are formedindependent from each other. In other words, the pair of the intakeports 8 are not combined with each other in the cylinder head 2, andpass independently from each other through the side surface of thecylinder head 2. This is called side injection (as opposed to topinjection wherein the intake ports pass through the top of the cylinderhead). Additionally, exhaust ports 9 are formed corresponding to theexhaust valves 6.

A generally cylindrical electromagnetic fuel injector valve 10 isdisposed at the lower surface section of the cylinder head 2. The lowersurface section is located adjacent to the side wall of the cylinder 3at the side of the intake valves 5. The fuel injector valve 10 isinstalled in a posture wherein its center axis is directed obliquelydownward. As shown in FIG. 2, the fuel injector valve 10 is disposedbetween the two intake valves 5.

A circular cavity combustion chamber 12 is formed at the top section ofthe piston 4 and is offset toward the intake valves 5. A spray axis ofthe fuel injector valve 10 is directed to this cavity combustion chamber12 when the piston is near the top dead center.

The intake ports 8 are respectively connected to a pair of intakepassages 14 a, 14 b which are formed independently at the side of anintake manifold 13. A butterfly-type valve air control valve 15 isdisposed in one intake passage 14 b so as to open or close the intakepassage 14 b. This air control valve 15 is controlled to open or closein accordance with the engine operating condition through a shaft 16 bya mechanical driving mechanism (not shown in the Figures).

When the air control valve 15 is closed, fresh air flows in only throughthe intake port 8 connected with the intake passage 14 a. This intakeport 8 connected to intake passage 14 a is not a helical port, butinstead is formed into a gradually curved port shape.

The basic operation of this internal combustion engine will now bedescribed. Homogenous charge combustion upon ignition of a homogenousair-fuel mixture formed inside the cylinder 3 is accomplished at a fullload of the engine when the air-fuel ratio is relatively small. Duringthis homogeneous combustion, the air control valve 15 is controlled tobe in an opened state so that fresh air is introduced into the cylinder3 through both intake ports 8. Because of this flow through both intakeports, a strong tumble stream (that is, vertical turbulence or avertical vortex) is formed inside the cylinder 3. Fuel is injected andsupplied into the cylinder 3 during intake stroke. This fuel ispositively dispersed inside the cylinder 3 under the action of thistumble stream, and therefore homogenization occurs in the cavitycombustion chamber 12.

In the low load operating region, stratified charge combustion iscarried out. Adequate ignition is made possible due to stratifioation ofthe air-fuel mixture. During stratified charge combustion, the aircontrol valve 15 is closed so that fresh air flows into the cylinder 3through only one intake port 8. Because of this, the tumble component isrelatively weak inside the cylinder 3, while a strong swirl stream isformed in a horizontal plane of the cylinder. During stratified chargelean combustion, fuel is injected from the fuel injector valve 10 towardthe cavity combustion chamber 12 in the latter half of the compressionstroke. This injected fuel moves to the side of the spark plug 7together with the swirl stream enclosed inside the cavity combustionchamber 12 at the top section of the piston 4 to form an ignitableair-fuel mixture around the spark plug 7. Combustion occurs by ignitionat the appropriate time.

Next, the configuration of the piston 4, particularly the configurationof the top section of the piston 4, will be described in detail withreference to FIGS. 3 to 5.

The top surface of piston 4 has a projection section 21 so that thecavity combustion chamber 12 occupies almost the entire part of thespace inside the cylinder 3 at top dead center. This projection section21 is constituted basically by five surfaces. The projection section 21is constituted by: an intake valve-side inclined surface 22 and anexhaust valve-side inclined surface 23 which are respectivelyconstituted of parallel planes which are generally parallel with the twoinclined surfaces 11 a, 11 b constituting the pent roof type combustionchamber 11; a top horizontal surface 30 which is formed band-shaped soas to connect the upper edge of the intake valve-side inclined surface22 and the upper edge of the exhaust valve-side inclined surface 23, andis constituted as a plane perpendicular to the center axis of the piston4; and a pair of conical side surfaces 24, 25 constituted respectivelyof conical planes each of which is coaxial with the outside circle ofthe piston 4 and gradually inclines.

Generally crescent-shaped intake valve-side horizontal surface 26 andexhaust valve-side horizontal surface 27 are respectively formed outsidethe above-mentioned intake valve-side inclined surface 22 and exhaustvalve-side inclined surface 23. The intake valve-side horizontal surface26 and exhaust valve-side horizontal surface 27 lie in one plane whichis perpendicular to the center axis of the piston 4, so that theycorrespond respectively to squish areas 2 a, 2 b (see FIG. 1) which areleft as flat planes at the opposite sides of the combustion chamber 11on the side of the cylinder head 2. Very narrow horizontal surfaces 28,29 are left between the arcuate lower edges of the conical side surfaces24, 25 and the outer peripheral edge of the piston 4 (see FIG. 5). Thesehorizontal surfaces 26, 27, 28, 29 lie in one plane and at the sameheight position. Further, in this embodiment, in consideration ofcompression ratios, a step section 34 exists between the lower edge 23 aof the exhaust valve-side inclined surface 23 and the exhaust valve-sidehorizontal surface 27. This step section 34 is constituted as a planewhich is inclined with respect to the exhaust valve-side inclinedsurface 23.

The cavity combustion chamber 12 is recessed through three surfaces,i.e., the top horizontal surface 30, the intake valve-side inclinedsurface 22, and the intake valve-side horizontal surface 26. The cavitycombustion chamber 12 is round in a plan view of the piston 4 and has adiameter larger than the radius of the piston 4. The bottom surface ofthe cavity combustion chamber 12 is in a plane perpendicular to thecenter axis of the piston 4 and is dish-shaped so that its innerperipheral side wall surface is gradually taperingly spread in an upwarddirection. The cross-sectional area of chamber 12 continually increasesas the chamber 12 approaches the top of the piston.

The outer periphery of the cavity combustion chamber 12 is locatedinside a pair of intake valve-side side ridgelines 31, 32, so that theintake valve-side inclined surface 22 is left in a band-shape at theopposite sides of the cavity combustion chamber 12. Similarly, a part ofthe outer periphery of the cavity combustion chamber 12 that is close tothe exhaust valves is located slightly one-sided and nearer to theintake valves than to an exhaust valve-side top ridgeline 33 (betweenthe top horizontal surface 30 and the exhaust valve-side inclinedsurface 23), so that a small portion of the top horizontal surface 30 isleft between the ridgeline 33 and the outer periphery of the cavitycombustion chamber 12. In other words, the outer periphery of the cavitycombustion chamber 12 does not intersect the respective ridgelines 31,32, 33 and is not cut out by these lines. Additionally, as shown in FIG.2, the spark plug 7 is disposed to enter the cavity combustion chamber12 at an outer peripheral section of the cavity combustion chamber whenthe piston 4 is at the top dead center.

An apex angle (see FIG. 5) of the conical side surfaces 24, 25 in theabove-mentioned projection section 21 is set as small as possiblekeeping the horizontal surface 28, 29 as narrow as it is in order thatthe ridgelines 35, 36 between the conical side surfaces 24, 25 and thetop horizontal surface 30 are located at the side of the outer peripheryof the piston 4. Because of this, when the piston 4 is at top deadcenter, a clearance produced between the conical side surfaces 24, 25and the combustion chamber 11 at the side of the cylinder head 2 is madevery small, so that the major part of the volume left inside thecylinder 3 is occupied by the cavity combustion chamber 12.

The structure of the top section of the piston 4 configured as discussedabove is formed symmetrical with respect to a diametrical line (lineIV-IV in FIG. 3), serving as a center, directed perpendicular to thepiston pin. The fuel injector valve 10 is located to inject fuel alongthis line.

In the above-described arrangement, the cavity combustion chamber 12 hasa round shape, and therefore swirl produced in the cylinder 3 duringstratified charge combustion is smoothly guided into the cavitycombustion chamber 12 and preserved with a sufficient intensity. Then,when the piston 4 comes near its top dead center position after fuel isinjected toward the cavity combustion chamber 12 at the latter half ofthe compression stroke, the three planes, i.e., the top horizontalsurface 30 surrounding the cavity combustion chamber 12, the intakevalve-side inclined surface 22, and the intake valve-side horizontalsurface 26 respectively approach corresponding planes at the side of thecylinder head 2, so that the cavity combustion chamber 12 is well sealedat its whole periphery. Accordingly, combustion proceeds and leakage ofthe swirl and air-fuel mixture is prevented. As a result, stablestratified charge lean combustion is made possible without forming theintake ports 8 as helical ports, and therefore maximum power output isprovided.

During homogeneous charge combustion, a tumble stream is formed insidethe cylinder 3 under the action of fresh air from the pair of the intakeports 8, and fuel injection is made in the intake stroke. Fuel suppliedinto the cavity combustion chamber is easily washed away by the tumblestream and is thus prevented from stagnating because the cavitycombustion chamber 12 is dish-shaped with its inner peripheral side wallsurface gradually taperingly spread at its upper section. The roundcavity combustion chamber 12 is located on the center line between thepair of intake ports 8 to which the tumble stream is concentrated.Accordingly, a homogeneous air-fuel mixture can be formed even at a highload, thereby making good homogeneous charge combustion possible.

Second Embodiment

FIGS. 6 to 8 will be used to describe a second embodiment of a piston 4according to this invention.

In this embodiment, the step section 34 between the exhaust valve-sideinclined surface 23 and the exhaust valve-side horizontal surface 27 isomitted, so that the lower edge 23 a of the exhaust valve-side inclinedsurface 23 reaches the exhaust valve-side horizontal surface 27.

In the arrangement of the second embodiment, as shown in FIG. 8, duringhomogeneous charge combustion (in which fresh air is introduced fromboth intake ports 8) a tumble stream as indicated by arrows can movesmoothly from the exhaust valve-side horizontal surface 27 to theexhaust valve-side inclined surface 23. (The flow will vary from thatshown in FIG. 8 as the position of the piston within the cylinderchanges.) In other words, attenuation of the tumble stream due tounevenness caused by the step section 34 in the first embodiment isreduced, thereby preserving the tumble component during the compressionstroke. As a result, HC (hydrocarbon) emissions at a fully opened statecan be reduced.

Third Embodiment

FIGS. 9 to 15 illustrate a third embodiment of the invention.

A variety of variable valve operating mechanisms have been hithertoproposed to variably control valve lift characteristics such as openingand closing timings, the operating angle, and the like, of intake valvesand exhaust valves of internal combustion engines, in accordance withengine operating conditions. For example, Japanese Utility ModelProvisional Publication No. 57-198306 and Japanese Patent ProvisionalPublication No. 6-185321 disclose a variable valve operating mechanismin which the valve operating angle can be varied under irregular speedrotation of a camshaft. Systems where two kinds of cams are selectivelyused and systems where the phase of a camshaft relative to a crankshaftis retarded or advanced, and other types of systems are known.

If a variable valve operating mechanism is intended to be applied to,for example, the intake valve side of a direct injection type internalcombustion engine, it becomes necessary to form a valve recess at thepiston top section in order to avoid interference between the intakevalve and the piston. However, if the valve recess is merely added, theinside and outside of the cavity combustion chamber may be brought intocommunication through the valve recess, and therefore performance duringstratified charge combustion is reduced.

In this embodiment, a known variable valve operating mechanism isprovided for the intake valves 5, so that the valve lift characteristicsof the intake valves can be variably controlled in accordance withengine operating conditions. Examples of valve lift characteristicsobtained by a variable valve operating mechanism are shown in FIGS. 9and 10. In FIG. 9, the opening and closing timings are retarded oradvanced while the operating angle is kept constant. In FIG. 10, theoperating angle increases or decreases while the valve center angle iskept constant.

As shown in FIGS. 11 to 15, this embodiment is similar to the FIG. 1embodiment except that in this embodiment, the intake valve-sideinclined surface 22 is not left as a plane surface due to the formationof valve recesses 31, 32 (discussed below) and the cavity combustionchamber 12, and therefore surface 22 is shown as an imaginary plane, asindicated by a dotted line in FIG. 14.

A pair of valve recesses 31, 32 is formed recessed corresponding to thevalve head sections of the intake valves 5 at the intake valve-sideinclined surface 22. These valve recesses 31, 32 are formed in arelatively shallow circular-shape and at the valve inclination angle.They are superposed on the cavity combustion chamber 12 so that each ofthem appears as a crescent shape. In this embodiment, a part of theouter periphery of these valve recesses 31, 32 reaches the tophorizontal surface 30, the conical side surfaces 24, 25, and the intakevalve-side horizontal surface 26. Recesses 31, 32 allow a large intakevalve lift amount.

The outer periphery of the cavity combustion chamber 12 is locatedinside a pair of imaginary side ridgelines produced between the conicalside surfaces 24, 25 and the intake valve-side inclined surface 22. Inother words, in the direction of the piston pin axis, the intakevalve-side inclined surface 22 is larger than the cavity combustionchamber 12. Similarly, a part of the outer periphery of the cavitycombustion chamber 12 that is near the exhaust valves is locatedslightly one-sided and nearer to the intake valves than exhaustvalve-side top ridgeline 33 (between the top horizontal surface 30 andthe exhaust valve-side inclined surface 23) so that a small width of thetop horizontal surface 30 is left between the ridgeline 33 and the outerperiphery of the cavity combustion chamber 12. In other words, the outerperiphery of the cavity combustion chamber 12 does not intersect theexhaust valve-side top ridgeline 33. Step section 34 can be used to varythe compression ratio. Additionally, as shown in FIG. 12, the spark plug7 is disposed to enter the cavity combustion chamber 12 and to belocated at the outer peripheral section of the cavity combustion chamberwhen the piston 4 is at the top dead center.

The ridgelines 35, 36 are located at the side of the outer periphery ofthe piston 4, and therefore the length of the top horizontal surface 30in the direction of the piston pin axis generally extends throughout theentire width of the piston, in the direction of the piston pin axis,where the pair of valve recesses 31, 32 is formed.

In this arrangement, the cavity combustion chamber 12 is round, andtherefore swirl produced in the cylinder 3 during stratified chargecombustion is smoothly guided in the cavity combustion chamber 12 andpreserved with a sufficient intensity. When the piston 4 comes near itstop dead center position after fuel is injected toward the cavitycombustion chamber 12 at the latter half of the compression stroke, thethree planes, i.e., the top horizontal surface 30 surrounding the cavitycombustion chamber 12, the intake valve-side inclined surface 22(actually the bottom surfaces of the valve recesses 31, 32), and theintake valve-side horizontal surface 26 respectively approach thecorresponding planes at the side of the cylinder head 2, so that thecavity combustion chamber 12 is well sealed along its entire periphery.Accordingly, combustion proceeds and leakage of swirl and air-fuelmixture from inside the cavity combustion chamber 12 is prevented. As aresult, stable stratified charge lean combustion is accomplished withoutforming the intake ports 8 as helical ports, and therefore maximum poweroutput is provided.

The top horizontal surface 30 between the cavity combustion chamber 12or the valve recesses 31, 32 and the exhaust valve-side inclined surface23 is maintained even though valve recesses 31, 32 are formed recessedand superposed on the cavity combustion chamber 12. As a result, adverseinfluence due to formation of the valve recesses 31, 32 is very small,and therefore sufficiently good stratified charge combustion can beensured.

As discussed above, the valve recesses are formed recessed at the intakevalve-side inclined surface, at which a part of the outer periphery ofthe cavity combustion chamber appears to be cut out by a valve recesses.However, the band-shaped top horizontal surface exists between the valverecesses and the exhaust valve-side inclined surface along almost theentire width of the piston, and therefore gas flow between the cavitycombustion chamber and the exhaust valve side is suppressed, therebypreventing stratified charge combustion from being degraded.

Fourth Embodiment

FIGS. 16 to 18 illustrate a fourth embodiment of the invention.

In this embodiment, the above-discussed step section 34 between theexhaust valve-side inclined surface 23 and the exhaust valve-sidehorizontal surface 27 is omitted, such that the lower edge 23 a of theexhaust valve-side inclined surface 23 reaches the exhaust valve-sidehorizontal surface 27.

In this embodiment, as shown in FIG. 18, during homogeneous chargecombustion in which fresh air is introduced from both intake ports 8,the tumble stream (as indicated by arrows) can more smoothly flow fromthe exhaust valve-side horizontal surface 27 to the exhaust valve-sideinclined surface 23. (The flow will vary from that shown in FIG. 18 asthe position of the piston within the cylinder changes.) In other words,attenuation of the tumble stream due to unevenness causes by stepsection 34 is reduced, thereby preserving the tumble component long intothe compression stroke. As a result, HC emissions are reduced.

Fifth Embodiment

FIGS. 19 to 24 illustrate a fifth embodiment of the invention.

The configuration of the piston 4 of this embodiment, particularly theconfiguration of the top section of the piston, will be discussed indetail with reference to FIGS. 19-22.

In this embodiment, the intake valve-side inclined surface 22 is left asonly a small part in the vicinity of the top section due to formation ofvalve recesses 31, 32 and the cavity combustion chamber 12. Therefore,the major part of the intake valve-side inclined surface 22 is animaginary plane, as indicated by a dotted line in FIG. 22. Additionally,the conical side surfaces 24, 25 are contiguous with each other,extending through the lower edge of the exhaust valve-side inclinedsurface 23, in this embodiment.

The apex angle of the conical side surfaces 24, 25 in the projectionsection 21 is very small, and therefore the conical side surfaces 24, 25rise steeply, as shown in FIG. 22. Along with this, the location of theridgelines 35, 36 between the conical side surfaces 24, 25 and the tophorizontal surface 30 approaches the outer peripheral side of the piston4. Because of this, clearance formed between the conical side surfaces24, 25 and the combustion chamber 11 at the side of the cylinder head 2is very small when the piston 4 is at top dead center, such that themajor portion of volume left in the cylinder 3 is occupied by the cavitycombustion chamber 12.

A piston standard horizontal surface 26 is formed at the outer peripheryof the projection section 21. This piston standard horizontal surface 26is constituted as one plane perpendicular to the center axis of thepiston 4 and is continuous throughout the entire periphery of the piston4. Parts at a thrust side and an anti-thrust side of this pistonstandard horizontal surface 26 correspond respectively to squish areas 2a, 2 b (see FIG. 19) left as flat surfaces at opposite sides of thecombustion chamber 11 at the side of the cylinder head 2, therebycontributing to production of squish.

The cavity combustion chamber 12 is recessed throughout the tophorizontal surface 30 and the intake valve-side inclined surface 22. Thecavity combustion chamber 12 is completely round in a plan view of thepiston 4 and has a diameter larger than the radius of the piston 4. Thebottom surface of the cavity combustion chamber 12 is in a planeperpendicular to the center axis of the piston 4 and chamber 12 isdish-shaped so that its inner peripheral side wall surface is graduallytaperingly spread in an upward direction. Additionally, the outerperiphery of the cavity combustion chamber 12 is located inside a pairof imaginary side ridgelines between the conical side surfaces 24, 25and the intake valve-side inclined surface 22. In other words, in thedirection of the piston pin axis, the intake valve-side inclined surface22 is larger than the cavity combustion chamber 12. In contrast, aportion of the outer periphery of the cavity combustion chamber 12 closeto the exhaust valves slightly extends over the exhaust valve-side topridgeline 33 between the top horizontal surface 30 and the exhaustvalve-side inclined surface 23, toward the exhaust valves. This isbecause, in this embodiment, the piston has a relatively small diameter(such as for a 1.8 liter, 4 cylinder engine). Additionally, as shown inFIG. 20, the spark plug 7 is disposed to enter the cavity combustionchamber 12 and is located at the outer peripheral section of the cavitycombustion chamber when the piston 4 is at top dead center.

The valve recesses 31, 32 are formed recessed corresponding to the valvehead sections of the intake valves 5 at the intake valve-side inclinedsurface 22. These valve recesses 31, 32 are formed in a relativelyshallow circular-shape and along the valve inclination angle, and aresuperposed on the cavity combustion chamber 12 such that each recessappears in a crescent shape. The reference character L in FIG. 22indicates the center axis of each intake valve 5. In this embodiment, aportion of the outer periphery of valve recesses 31, 32 reaches thevicinity of the imaginary side ridgelines between the conical sidesurfaces 24, 25 and the inclined surface 22. Ridgelines 35, 36 at theopposite ends of the top horizontal surface 30 in the direction of thepiston pin axis are located close to the outer periphery of the piston4, and therefore the ridgelines 35, 36 are located outside the end edgeposition of each valve recess 31, 32 in the direction of the piston pinaxis as viewed in the direction of the piston pin axis. Valve recesses31, 32 have a depth such that the recesses are not lower than the pistonstandard horizontal surface 2 in the axial direction of the piston 4. Inother words, as apparent from FIG. 22, valve recesses 31, 32 are notdepressed below the piston standard horizontal surface 26.

The structure of the top section of the piston 4 configured as discussedabove is symmetrical with respect to a diametrical line (or a lineXXII—XXII in FIG. 21). serving as a center and perpendicular to thepiston pin. Fuel injector valve 10 is located to inject fuel along thisline serving as a symmetry axis.

The valve recesses 31, 32 are formed recessed at the intake valve-sideinclined surface 22 of the projection section 21, as discussed above,and therefore projection section outer peripheral section 21 a is leftin the shape of an arcuate dam, along the outer periphery of the valverecesses 31, 32. Valve recesses 31, 32 are gradually lowered inaccordance with the inclination of the intake valve-side inclinedsurface 22 in a direction toward the intake valve side of the piston 4(in a rightward direction in FIG. 21). The projection section 21ultimately becomes the same in height as the piston standard horizontalsurface 26 and disappears. Here, a line connecting the tip enddisappearing point of this projection section 21 and the center of thecavity combustion chamber 12 forms an angle (see FIG. 21) of not largerthan 45° (relative to the above-mentioned line XXII—XXII). Thisminimizes gas flow from the cylinder outer peripheral section throughthe valve recesses into the cavity combustion chamber.

In the above-discussed arrangement, the cavity combustion chamber 12 isround and therefore swirl produced in the cylinder 3 during stratifiedcharge combustion is smoothly guided into the cavity combustion chamber12 and preserved with a sufficient intensity. When the piston 4 comesnear its top dead center position after fuel is injected toward thecavity combustion chamber 12 at the latter half of the compressionstroke, the respective surfaces of the projection section 21respectively approach the corresponding surfaces of the cylinder head 2as indicated by a dotted line Q in FIG. 19, so that the cavitycombustion chamber 12 is well sealed at its entire periphery.Accordingly, combustion proceeds and leakage of swirl and the air-fuelmixture inside the cavity combustion chamber 12 is prevented fromleaking to the outside.

Valve recesses 31, 32 are formed depressed and superposed on the cavitycombustion chamber 12. However, valve recesses 31, 32 are not depressedbelow the piston standard horizontal surface 26. As a result, when thepiston 4 is near top dead center, a swirl component along the outerperipheral section of the cylinder 3 does not enter the valve recesses31, 32 and instead flows on the piston standard horizontal surface 26 atthe outer peripheral section, thereby suppressing entrance of this swirlcomponent into the cavity combustion chamber 12. Since the side sectionof the valve recesses 31, 32 is surrounded by the projection sectionouter peripheral section 21 a, gas flow flowing on the piston standardhorizontal surface 26 is suppressed from flowing into the valve recesses31, 32. Further, since the top horizontal surface 30 extends in thedirection of the piston pin axis between a portion of the cavitycombustion chamber 12 and valve recesses 31, 32 and the exhaustvalve-side inclined surface 23, gas flow from the exhaust valve side ofthe cylinder 3 toward the cavity combustion chamber 12 is interruptedand weakened. Accordingly, swirl flow and fuel inside the cavitycombustion chamber 12 is not disturbed by gas flow from outside of thecavity combustion chamber 12, and adverse influence due to formation ofthe valve recesses 31, 32 is minimized, thereby ensuring good stratifiedcharge combustion.

Additionally, during homogeneous charge combustion, a bumble stream isformed inside the cylinder 3 under the action of fresh air from theintake ports 8, and fuel injection is made in the intake stroke. Fuelsupplied into the cavity combustion chamber 12 is easily washed away bythe tumble stream and is thus prevented from stagnating because thecavity combustion chamber 12 is dish-shaped such that its innerperipheral side wall surface is gradually taperingly spread at its uppersection. The round-shaped cavity combustion chamber 12 is dish-shapedsuch that its inner peripheral side wall surface is gradually taperinglyspread at its upper section. The round-shaped cavity combustion chamber12 is located on the center line (the line XXII—XXII in FIG. 21) betweenthe pair of intake ports 8 to which the tumble stream is concentrated.Accordingly, a homogeneous air-fuel mixture is formed even at a highload, thereby making good homogeneous charge combustion possible.

FIG. 23 shows results of experiments examining the relationship betweenthe depth of the valve recesses 31, 32 and the combustion stabilityduring stratified charge combustion. As shown by FIG. 23, the combustionstability is abruptly degraded if the vale recesses 31, 32 are recessedbelow the piston standard horizontal surface 26. Conversely, degradationin combustion stability, due to the valve recesses 31, 32, is small ifthe depth of the vale recesses does not exceed the piston standardhorizontal surface 26.

FIG. 24 shows results of experiments which examine the effect of movingthe position of ridgelines 35, 36 along the piston pin direction (thatis, the length of the top horizontal surface 30). In FIG. 24, one curveindicates HC emission amount in the embodiment of FIG. 36 wherein thetop horizontal surface 30 is longer than the combined length of thevalve recesses 31, 32 in the piston pin direction. In the FIG. 36embodiment, the ridgelines 35, 36 are placed near the cylinder wall,that is, closer to the cylinder wall than valve recesses 31, 32, in thepiston pin direction. The other curve in FIG. 24 indicates HC emissionamount for the FIG. 21 embodiment. In the FIG. 21 embodiment, ridgelines35, 36 are inside the ridges of the valve recesses 31, 32 in the pistonpin direction. In other words, in the FIG. 21 embodiment, the length oftop horizontal surface 30 is shorter than the combined length of thevalve recesses 31, 32. As apparent from a comparison between these twocurves, lowering of the HC emission amount can be achieved bypositioning the ridgelines 35, 36, on the periphery of piston 4.

This embodiment provides for a large valve lift amount at top deadcenter. Also, the swirl component turning along the cylinder outerperipheral section is prevented from flowing into the cavity combustionchamber through the valve recess during stratified charge combustion.

Sixth Embodiment

FIGS. 25 to 31 illustrate a sixth embodiment of the invention.

According to the sixth embodiment, the fuel injection valve injects thefuel toward a portion of the swirl upstream of the ignition plug,thereby causing the injected fuel to be stratified at a portion of theswirl upstream of the ignition plug to enable the fuel spray to besurely ignited, which results in the further improvement of thestability of the super-lean burning. Moreover, this reduces the amountof fuel directly injected, thereby reducing the amount of fuel depositedto the ignition plug, which results in improvement of the coldstart-ability of the internal combustion engine.

FIGS. 25 to 27 show a four-valve type spark ignition internal combustionengine having two intake valves, and two exhaust valves. A first intakeport 108 and a second intake port 109 are independently branched from anintake manifold 107 and open to a pent roof-like combustion chamber 106defined by a piston 103 and a cylinder head 102, and are opened andclosed through intake valves 104A, 104B.

In the cylinder head 102, opposed to the first and second intake ports108, 109 are exhaust ports 113, 113, which are opened and closed byexhaust valves 105A, 105B. In the center of the inner periphery of thecylinder head 102, surrounded by the intake valves 104A, 104B and theexhaust valves 105A, 105B, an ignition plug 115 is provided. Theignition plug 115 is arranged on the center line C of the cylinder 101.

The first and second intake ports 108, 109 independently branch from theintake manifold 107, and each is formed as a straight port. The firstintake port 108 and the second intake port 109 are separated at apredetermined interval.

At an intermediate portion in the first intake port 108 and the secondintake port 109, first and second swirl control valves 110, 111 areprovided. These valves are connected through an opening and closingshaft 112, and are opened and closed in synchronization with each otherby an actuator (not shown in the Figures).

The second swirl control valve 111 disposed on the second intake port109 is closed to close the second intake port 109 as shown by a brokenline in FIG. 25 when the valve is fully closed, that is, _(α)=0°, whereas the second intake port 109 is capable of communicating the intakemanifold 107 with the combustion chamber 106 when the valve 111 is fullyopened, that is, _(α)=90°, as shown by reference numeral 111′ in FIG.25.

As shown in FIG. 26, a portion 110A is formed on the first swirl controlvalve 110 disposed in the first intake port 108, so as to throttle thepassage sectional area while communicating the intake manifold 107 withthe combustion chamber 106. In this embodiment, the first swirl controlvalve 110 is shaped like a semi-circle. Therefore, when the valve isfully closed, that is, _(α)=0°, the intake air passes therethrough overa predetermined passage sectional area between the portion 110A and thefirst intake port 108. In this embodiment, the portion 110A reduces theflow area of port 108 by about 50%, however this percentage can bevaried. Providing flow at only the outermost portion of port 108 (thelower portion of 108 in FIG. 26) contributes to the generation of swirl.

the first intake port 108 is capable of communicating the intakemanifold 107 with the combustion chamber 106 without throttling of thepassage sectional area when the valve is fully opened, that is,_(α)=90°.

The opening and closing shaft 112 can be controlled to be locked at anarbitrary position between the fully open position and the fully closeposition of the first and second swirl control valves 110, 111 by theactuator.

A circular concave portion 130 having a predetermined depth is formed onthe top surface 103A of the piston 103 opposed to the combustion chamber106. Piston 103 is connected to a crankshaft through a piston pin 117and a connecting rod 116. Portion 130 is just below the ignition plug115 toward the intake valves 104A, 104B up to the periphery edge of thepiston 103 and provides swirl flow at the inner periphery of the concaveportion 130 to cause the swirl flow to be stratified and then led to theignition plug 115, at the time of stratified burning.

As shown in FIG. 26, a fuel injection valve 114 is disposed along thecenter line C of the cylinder 101 between the first intake port 108 andthe second intake port 109. An injection hole 114A side of the fuelinjection valve 114 is inserted through an opening portion 120 formed onthe cylinder head 102, and the injection hole 114A is arranged at apredetermined position so as to face the combustion chamber 106.

An axial line Jc of the fuel spray jetting through the injection hole114A of the fuel injection valve 114 is coaxial with the axis of theinjection valve 114, in this embodiment. In FIG. 25, the axial line Jcof the fuel spray is set so as to cross with substantially the center ofthe concave portion 130 formed on the piston top surface 103 A, and asshown in FIG. 26, the axial line Jc of the fuel spray is set so as to becoaxial with the center line C of the cylinder 101. Therefore, as shownin FIG. 25, the fuel injection valve 114 is supported on the cylinderhead 102 while being inclined at a slight angle with respect to thefirst and second intake ports 108, 109.

A spray recess 121 is disposed on the cylinder had near the injectionhole 114A in the fuel injection valve 114. The design of this recesswill now be described.

The swirl flow 140 is generated in the combustion chamber 106, andstratification is realized by introducing the fuel spray into the swirlflow 140 at the time near stratified burning. It is desirable that theentire injection hole 114A be exposed to the combustion chamber 106.However, the outer periphery of the fuel spray should be prevented fromcontacting the cylinder head 102. When the upper portion of theinjection hole 114A is embedded in the cylinder head 102, as shown inFIG. 25, a part 140′ of the swirl flow 140 should be introduced to theinjection hole 114A. Concave spray recess 121 provides for this (asshown in FIG. 27) and also prevents the fuel spray from being deposited.

As shown in FIG. 31, if a concave portion 122 is provided to prevent thefuel spray from being deposited on the cylinder head covering the upperpart of the injection hole, a deposit, shown by hatching in FIG. 31, isunexpectedly deposited on an inner periphery of the portion 122,upstream of the swirl flow 140.

On the contrary, the curved shape of concave spray recess 121 preventsthe outer periphery of the fuel spray from contacting the cylinder headand also guides a part 140′ of the swirl flow 140 in the combustionchamber 106 to the vicinity of the injection hole 114A. This recess onthe cylinder head 102 covering the upper part of the injection hole 114Aprevents the deposit from occurring upstream of the swirl flow, whichresults in improvement of reliability and endurance and reduces thesmoke generation amount and the HC discharge amount, improving emissionperformance.

The fuel injection valve 114 injects fuel in a cone-shaped manner havinga predetermined angle between 50° to 80° and the pressure of thesupplied fuel is set to a relatively low pressure of, for example, 5MPa.

Operations of the above-described construction will now be described.These operations are summarized in FIGS. 28 and 29.

At the time of stratified burning in which fuel consumption is reduced,the opening and closing shaft 112 is driven to a position where thevalve is fully closed, that is, _(α)=0°, thereby causing the first andsecond swirl control valves 110, 111 to close.

When the valve opening is 0°, the second swirl control valve 111 fullycloses the second intake port 109, while the first swirl control valve110, having the portion 110A, throttles the passage sectional area ofthe first intake port 108, thereby allowing passage of the intake airbetween the portion 110A and the inner wall.

Thus, during the air intake stroke, air intake is carried out onlythrough the first intake port 108 having the throttled passage sectionalarea, thereby causing (as shown by arrow 140 in FIG. 26) the swirl flow140 in the anti-clockwise direction as viewed from the rear surface ofthe piston 103 to be generated in the combustion chamber 106, whichcauses the swirl flow 140 to be maintained in the concave portion 130formed in the top surface 103A of the piston 103 during the subsequentcompression stroke.

Then, at the time of stratified burning, fuel is injected from the fuelinjection valve 114 toward the concave portion 130 in the state shown inFIG. 25 at the end of the compression stroke, thereby realizing ignitionby means of ignition plug 115, arranged opposed to the concave portion130, to realize super-lean burning in which the air-fuel ratio A/F isover 40.

On the other hand, at the time of homogeneous burning in which theengine torque is increased, the opening and closing shaft 112 is drivento a position where the valve opening is 90°, thereby causing the firstand second swirl control valves 110, 111 to be opened.

When the valves are fully opened, the first and second swirl controlvalves 110, 111 fully open the first and second intake ports 108, 109,thereby allowing the intake air to pass through the two independentintake ports 108, 109.

Therefore, during the intake stroke, air is suctioned equally throughthe first and second intake ports 108, 109, thereby causing tumble flowto be generated in the combustion chamber 106. At the time ofhomogeneous burning, the fuel is injected from the fuel injection valve114 during the intake stroke, thereby causing tumble flow in thecombustion chamber 106 to cause the fuel spray (corresponding to anair-fuel ratio A/F near the stoichiometric air-fuel ratio) to behomogenized, which ensures good ignition by the ignition plug 115. Dueto the air-fuel ratio A/F being near the stoichiometric air-fuel ratio,the engine torque is larger than during stratified burning. Inparticular, when the throttle is fully opened, an output equivalent toor more than a conventional MPI type internal combustion engine can beobtained.

During the intake stroke at the time of homogeneous burning, air issuctioned through the first and second intake ports 108, 109, comprisingtwo straight ports, thereby enabling the engine output to be improved,compared with the case in which one of two independent intake ports is ahelical port. This also eliminates the need for the addition of acomplicated mechanism such as a variable valve timing system, like aconventional example. This makes the construction of a direct injectiontype spark ignition internal combustion engine simple and reducesproduction cost, and also provides both an output equivalent to a MPItype internal combustion engine and super-lean burning due to stratifiedburning in one engine.

In this embodiment, intermediate regions exist between stratifiedburning and homogeneous burning. A stratified lean burning condition hasan air-fuel ratio (A/F=30) less than the above-mentioned stratifiedburning, and a homogeneous lean condition has an air-fuel ratio (A/F=20)more than the above-mentioned homogeneous burning. At the time ofstratified lean burning or homogeneous lean burning, the opening of thefirst and second swirl control valves 110, 111 is set to an intermediateangle of about 45°.

According to experiments, the relationship between air-fuel ratio A/F,or the burning condition, and the swirl ratio S and the tumble ratio Tis shown in FIG. 29. As shown, at the time of stratified burning of thesuper-lean air-fuel ratio A/F, swirl flow is primarily generated tocause the stratification of the fuel spray to be accelerated. Therefore,it is desirable that the swirl ratio S be increased, where as the tumbleratio T be reduced. Accordingly, the first and second swirl controlvalves 110, 111 are closed, thereby causing strong swirl flow to begenerated.

On the other hand, when the air-fuel ratio A/F is on the rich side nearthe stoichiometric air-fuel ration, generating primarily tumble flowenables the fuel spray injected during the intake stroke to behomogenized. Therefore, it is desirable that the tumble ratio beincreased, whereas the swirl ratio S be reduced. Accordingly, the firstand second swirl control valves 110, 111 are opened, thereby causingstrong tumble flow to be generated by the two straight ports.

Since both tumble and swirl contribute to homogenization andstratification of the fuel spray in the intermediate regions ofstratified burning and homogeneous burning, the opening of the first andsecond swirl control valves 110, 111 is set to an intermediate openingof about 45°, thereby causing both swirl flow and tumble flow to begenerated. When the valve opening is about 45°, the air is suctioned atan intake air amount corresponding to the valve opening through thefirst and second intake ports 108, 109. Atmospheric air flows throughthe two intake ports 108, 109, thereby causing tumble flow to begenerated in the combustion chamber 106. Also, intake air flow of thefirst intake port 108 on the side of the first swirl control valve 110,having the portion 110A, is larger than that of the second intake port109 on the side of the second swirl control valve 111, thereby causingswirl to be generated in the combustion chamber based on the differencebetween the intake air flow amounts of the first and the second intakeports.

Also, in the homogeneous lean region, homogeneous burning of theair-fuel ratio of 20 is carried out with the first and second swirlcontrol valves 110, 111 set to an intermediate angle, and in thestratified lean region stratified burning of an air-fuel ratio of 30 iscarried out.

In the stratified lean burning region, after the first fuel injection iscarried out during the intake stroke, a second fuel injection is carriedout at the end of the compression stroke, as in the case with stratifiedburning, thereby providing stable lean burning in which the air-fuelratio A/F is about 30. As a result, fuel is injected at the lean burningside air-fuel ratio during the intake stroke in the homogeneous burningregion, which provides burning similar to the above-mentionedhomogeneous burning.

When the burning condition switches from stratified burning tohomogeneous burning, or vice versa, the burning condition is changedfrom stratified burning to homogeneous burning by changing-over theburning condition between stratified lean burning and homogeneous leanburning with the first and second swirl control vales 110, 111 set to anintermediate opening, thereby preventing excessive torque fluctuation(torque increase) when the burning condition is changed-over betweenstratified burning and homogeneous burning. This provides stable burningbetween super-lean burning and normal burning near the stoichiometricair-fuel ratio.

Each first and second swirl control valve 110, 111 is set to anintermediate angle of about 45° (which is varied depending on theoperating condition): therefore, the openings of the first and secondswirl control valves 110, 111 performing homogeneous lean burning andstratified lean burning are set to different values, respectively, atthe time of transition from stratified burning to homogeneous burning,and at the time of transition from homogeneous burning to stratifiedburning. The openings of the first and second swirl control valves 110,111 can be controlled continuously depending on the transition of theburning condition or the change of the air-fuel ratio A/F. Variablycontrolling the first and second swirl control valves 110, 111 disposedon independent first and second intake ports 108, 109 enables the ratioof the swirl and the tumble to be set to an optimum value correspondingto each of the burning conditions, thereby resulting in improvement ofthe output performance and the burning performance while reducing thesmoke generation amount and the HC discharge amount, to provideimprovements in emission characteristics.

FIG. 30 illustrates the relationship between the fuel injection angle ofthe fuel injection valve 114, and the burning stability and the smokegeneration amount of this direct injection type internal combustionengine.

As described above, when the fuel is injected by the use of pressurizedfuel of a relatively low pressure of about 5 MPa, the realize stablestratified burning by atomizing the fuel, experiments were conductedrelated to the relationship between the fuel injection angle and theburning stability (e.g., torque fluctuation amount) and the smokegeneration amount, in the case of atmospheric pressure. The results areshown in FIG. 30.

When the fuel injection angle is less than about 50°, the amount of fueldeposited to the concave portion 130 formed on the piston top surface103A is increased, thereby increasing the smoke generation amount. Thisalso increases the air-fuel ratio A/F near the ignition plug 115 tounexpectedly deteriorate the burning stability.

On the other hand, when the fuel injection angle exceeds about 90, thesmoke generation amount is lowered; however, the fuel spray flows out ofthe concave portion 130, thereby disabling smoothly stratified burningto unexpectedly deteriorate the burning stability again.

Therefore, setting the fuel injection angle of the fuel injection valve114 to a value between 50° to 90° in the case of atmospheric pressurerealizes burning stability while reducing the smoke generation amount atthe time of stratified burning, which enables operability and emissionperformance together.

Seventh Embodiment

FIGS. 32 to 35 illustrate a seventh embodiment and modificationsthereof. In FIGS. 32 and 33, the axial line Jc of the fuel injection ofthe fuel injection valve 114 is deflected to the side of the firstintake port 108 by a predetermined angle x with respect to the axialline C of the cylinder 101, and an electrode 115A of the ignition plug115 is inserted into an inner periphery of the concave portion 130formed on the piston top surface 103A in compression top dead center103′ of the piston 103. The other details are the same as those ofprevious embodiment(s).

When the piston 103 is at the compression top dead center 103′, theelectrode 115A of the ignition plug 115 is inserted into the innerperiphery of the concave portion 130 of the piston top surface 103A,causing the ignition plug 115 to be inserted into the swirl flowgenerated due to the suction through the first intake port 108, at thetime of stratified burning, which enables the stratified fuel spray tobe ignited. This results in improvement of the stability of thesuper-lean burning due to stratified burning.

Deflecting the axial line Jc of the fuel spray of the fuel injectionvalve 114 to the side of the first intake port 108 enables the injectedfuel to be stratified at a portion of the swirl upstream of the ignitionplug 115, and reduces the fuel required to be directly injected to theignition plug 115, which in turn reduces the fuel deposited on theignition plug 115 and results in improvement in the cold start-abilityof the internal combustion engine.

In FIG. 34, the axial line Jc of the fuel injection of the fuelinjection valve 114 is offset to the side of the first intake port 108by a predetermined amount L with respect to the axial line C of thecylinder 101. Other details are the same as those of previousembodiments.

Offsetting the axial line Jc of the fuel spray of the fuel injectionvalve 114 to the side of the first intake port 108 reduces the fuelrequired to be directly injected to the ignition plug 115, and causesthe fuel spray injected from the fuel injection valve 114 to bestratified at a portion of the swirl upstream of the ignition plug 115.This improves stability during stratified burning and reduces the fueldeposited on the ignition plug 115, which in turn results in improvementof cold start-ability of the internal combustion engine. The offsetamount L is set to a predetermined value such that the injected fuelspray does not leak through the concave portion 130 of the piston 103.

In FIG. 35, the offset amount L is applied to the ignition plug 115, andthus the ignition plug 115 is offset to the side of the second intakeport 109 by a predetermined amount L with respect to the axial line ofthe cylinder 101. The offset amount L is a predetermined value and isset such that the injection plug 115 can be opposed to the innerperiphery of the concave portion 130.

Offsetting the ignition plug 115 to the side of the second intake port109 reduces the fuel required to be directly injected to the ignitionplug 115, and causes the fuel spray injected from the fuel injectionvalve 114 to be stratified at a portion of the swirl upstream of theignition plug 115, and thus reduces the fuel deposited on the ignitionplug 115, which in turn results in improvement of the cold start-abilityof the internal combustion engine. This also improves stability duringstratified burning.

The entire contents of Japanese Patent Applications P9-135269 (filed May26, 1997), P9-137369 (filed May 28, 1997) P9-132673 (filed May 23,1997). P9-129053 (filed May 20, 1997, and Press Information entitled“Nissan Direct-Injection Engine” (Document E1-2200-9709 of Nissan MotorCo., Ltd. of Tokyo, Japan) are incorporated herein by reference.

Although the invention has ben described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the above teachings. For example, the cavity combustion chambercan be oval in shape, with a major axis along the thrust-antithrustdirection of the piston. The scope of the invention is defined withreference to the following claims.

What is claimed is:
 1. An internal combustion gasoline engine,comprising: a cylinder; a cylinder head at a head of the cylinder; apiston within the cylinder; a fuel injector to inject fuel in the formof gasoline directly into the cylinder; an air intake assembly tointroduce air through the cylinder head into the cylinder to generateswirl flow for stratified charge combustion and to generate tumble flowfor homogeneous charge combustion by controlling air flow into thecylinder; and an ignition plug, in the cylinder head, to ignite anair-fuel mixture in the cylinder; wherein the piston includes a firstincline surface approximately parallel to an intake-side inclinedsurface of the cylinder head; a second inclined surface approximatelyparallel to an exhaust-side inclined surface of the cylinder head; sidesurfaces connected to the first and the second inclined surfaces; and acavity combustion chamber recessed in the first inclined surface, thecavity combustion chamber having an increasing cross sectional area asthe top of the piston is approached.
 2. An engine as set forth in claim1, wherein the air intake assembly includes two straight intake ports.3. An engine as set forth in claim 2, wherein the two straight intakeports pass through the side of the cylinder head.
 4. An engine as setforth in claim 1, wherein the first inclined surface, the secondinclined surface, the side surfaces and the cavity combustion chamberare formed symmetrical with respect to a line which is perpendicular toa piston pin.
 5. An engine as set forth in claim 1, wherein the cavitycombustion chamber is recessed inside border liens of the first inclinedsurface and the side surfaces.
 6. An engine as set forth in claim 1,wherein the piston further comprises: a horizontal surface formed on anouter region of the piston, the horizontal surface being perpendicularto the axis of the piston; and a step section located on a boarder ofthe horizontal surface and the second inclined surface.
 7. An engine asset forth in claim 1, wherein the cavity combustion chamber is formedplate-shaped, with a substantially flat bottom and a side taperedexpanding upward.
 8. An engine as set forth in claim 1, wherein thepiston further comprises a horizontal surface formed on the entirecircumference of the piston.
 9. An engine as set forth in claim 1,wherein the cavity combustion chamber is a substantially perfect circlein plan view.
 10. An engine as set forth in claim 1, wherein the sidesurfaces have the shape of part of a cone.
 11. An engine as set forth inclaim 1, wherein the piston further comprises a bow-shaped squish areaon the intake side.
 12. An engine as set forth in claim 1, wherein thepiston further comprises a bow-shaped squish area on the exhaust side.13. An engine as set forth in claim 1, wherein at least one of the twostraight intake ports has a swirl control valve.
 14. An engine as setforth in claim 2, wherein both straight intake ports have swirl controlvalves.
 15. An engine as set forth in claim 14, wherein one of the swirlcontrol valves is notched to allow passage of air even when said one ofthe swirl control valves is shut.
 16. An engine as set forth in claim13, wherein the swirl control valves is shut for stratified chargecombustion and opened for homogeneous charge combustion.
 17. An engineas set forth in claim 2, wherein the intake ports are curved to thecenter of the cylinder in a view from a plane perpendicular to thepiston axis.
 18. An engine as set forth in claim 1, wherein the fuelinjector directs fuel along an axis directed at the center of the cavitycombustion chamber when the piston is near top dead center.
 19. Anengine as set forth in claim 1, wherein the piston further comprises aflat plane between top ends of the first and the second inclinedsurfaces, the flat plane perpendicular to the axis of the piston.
 20. Anengine as set forth in claim 1, wherein the cavity combustion chamberintersects with the second inclined surface.
 21. An engine as set forthin claim 1, wherein the cavity combustion chamber does not intersectwith the second inclined surface.